encoding/json/internal/jsontest: rename testdata to _embed

Various tools assume that testdata directories are unnecessary if you
don't need to run tests. This includes cmd/internal/bootstrap_test,
which runs go install std on a GOROOT excluding testdata directories.
Since this is an importable package rather than a test, to avoid
breaking that case we must not actually name the directory testdata.

We may want to reevaluate whether we want a dedicated package with
embedded testdata at all, but for now the simple fix for the currently
broken longtest builders is to simply rename the directory.

For #71497.

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Change-Id: Ic14ce7b06dca36cc758f99374da378c46a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/779302
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
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src/encoding/json/internal/jsontest/testdata.go

@@ -25,7 +25,13 @@ import (
 // by other packages such that the location of testdata may change relative
 // to the working directory of the test itself.
 //
-//go:embed testdata/*.json.zst
+// Various tools assume that testdata directories are unnecessary if you don't
+// need to run tests. This includes cmd/internal/bootstrap_test, which runs go
+// install std on a GOROOT excluding testdata directories. Since this is an
+// importable package rather than a test, to avoid breaking that case we must
+// not actually name the directory testdata.
+//
+//go:embed _embed/*.json.zst
 var testdataFS embed.FS
 
 type Entry struct {
@@ -43,7 +49,7 @@ func mustGet[T any](v T, err error) T {
 
 // Data is a list of JSON testdata.
 var Data = func() (entries []Entry) {
-	fis := mustGet(fs.ReadDir(testdataFS, "testdata"))
+	fis := mustGet(fs.ReadDir(testdataFS, "_embed"))
 	slices.SortFunc(fis, func(x, y fs.DirEntry) int { return strings.Compare(x.Name(), y.Name()) })
 	for _, fi := range fis {
 		var entry Entry
@@ -57,7 +63,7 @@ var Data = func() (entries []Entry) {
 
 		// Lazily read and decompress the test data.
 		entry.Data = sync.OnceValue(func() []byte {
-			filePath := path.Join("testdata", fi.Name())
+			filePath := path.Join("_embed", fi.Name())
 			b := mustGet(fs.ReadFile(testdataFS, filePath))
 			zr := zstd.NewReader(bytes.NewReader(b))
 			return mustGet(io.ReadAll(zr))